# Quality Control (stability) Chart for CMM's - How the control limits are to be calculated

#### Nihls

##### Registered
Hello,

We want to keep the stability of our coordinate measuring machines under control using a quality control card.

CMM: Zeiss Prismo Ultra
Measuring standard: KMG Check

It is not clear to me how the control limits are to be calculated. Does anyone have any experience in this regard?

Best regards

Elsmar Forum Sponsor

#### malasuerte

##### Involved In Discussions
Hello,

We want to keep the stability of our coordinate measuring machines under control using a quality control card.

CMM: Zeiss Prismo Ultra
Measuring standard: KMG Check

It is not clear to me how the control limits are to be calculated. Does anyone have any experience in this regard?

Best regards

Sorry - you are asking HOW to calculate control limits? You probably should take a stats class so you get full comprehension.

But in short:
1. First calculate the Center Line. The Center Line equals either the average or median of your data.
2. Second calculate sigma. The formula for sigma varies depending on the type of data you have.
3. Third, calculate the sigma lines. These are simply ± 1 sigma, ± 2 sigma and ± 3 sigma from the center line.

+ 3 sigma = Upper Control Limit (UCL)
- 3 sigma = Lower Control Limit (LCL)

#### Steve Prevette

##### Deming Disciple
Staff member
Super Moderator
I have a fair amount of material posted here at the Cove on implementing Statistical Process Control. If you have a set of data you are willing to share, I am willing to run an initial chart for you and post it here as an example.

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